Well known is a method for producing thin sheets having thicknesses of from about 0.076 to about 1.0 mm (0.003 to 0.04 inch) and made of titanium (Ti), zirconium (Zr) and alloys thereof (see the U.S. Pat. No. 2,985,945 published May 30, 1961). The method includes the steps of preparing a card blank, assembling a plurality of the blanks into a pack in an outer sheath (a steel case), heating the pack up to about 730-757° C. (from about 1345 to 1395° F.), hot rolling the pack, annealing the pack, cold rolling the pack at a reduction of from 10 to 60%, heat treating the pack, end cropping and end trimming the pack and separating the trimmed pack into component sheets, and finishing the sheets. The method allows to obtain required mechanical properties of the sheets in longitudinal and transverse directions by maintaining optimum temperature-deformation conditions of the process. The produced sheets have a grain size of 4 to 6 μm (microns) and greater. This method may be considered as the prior art closest to the methods claimed in the present invention.
However, the processing of high-strength alloys in the suggested temperature range is difficult and causes formation of microcracks and breaks in the processed material. In addition, the sheets produced by the above-described method can be used to form articles of a complex shape by superplastic forming (SPF) only at high temperatures (900-960° C.), which significantly complicates the technological process and makes the produced articles more expensive. Decrease of the SPF temperature below 800° C. causes an abrupt increase of stresses during deformation.
Also known from the prior art (see the U.S. Pat. No. 3,492,172 of Jan. 27, 1970) is a method for producing strips of a metal selected from the group consisting of commercially pure titanium, alpha stabilized alpha type titanium base alloys and alpha stabilized alpha-beta type titanium base alloys, which comprises: (1) unidirectionally hot rolling a body of said metal to reduce said body to an elongated hot band, said rolling being initiated at a temperature requiring a substantial amount of said reduction to occur in the alpha-beta field of said metal; (2) heating said hot band at a temperature above the beta transus of said metal to completely transform the crystal structure of said metal to the beta phase; (3) rapidly cooling said hot band from said temperature above the beta transus of said metal to a temperature below said beta transus to produce acicular type microstructure in the metal; and (4) subjecting said rapidly cooled hot band to the steps of rolling and annealing at temperatures below said beta transus to produce an elongated strip having a substantially completely recrystallized microstructure.
A method for manufacturing thin sheets of strength and high-strength titanium-based alloys is also known in the prior art (see the Russian Patent No. RU 2,179,899, IPC7 B21B 1/38, published on Feb. 27, 2002 and assigned to the present applicant). This method includes the steps of preparing card blanks, assembling the blanks into a pack in a steel case, heating the pack up to 880° C. and hot rolling the pack at a reduction rate of 60%, annealing the pack at the temperature of 770° C. for 30 min, straightening the pack, disassembling the pack into separate sheets, and finishing the sheets.
This method allows to obtain the sheets having α-phase grain sizes of 2-4 μm in their microstructure, which are quite sufficient for producing articles from these sheets by the SPF at temperatures of 900-960° C. This is an optimum temperature range in order to obtain necessary values of flow stress and elongation at a strain rate of from 10−3 to 10−4 sec−1.
However, decrease of the SPF temperature below 800° C. causes an abrupt increase in flow stresses up to 75 MPa (for a true deformation value of 1.1) and the sheets produced by this known method are therefore not suitable for the SPF at temperatures below 800° C.
The article manufacturing process using the SPF is commonly performed in special furnaces into which dies are placed and heated up to a deformation temperature of 900-960° C. A heated inert gas which creates a formation strain needed to shape the article is supplied under pressure to a workpiece through channels made in an upper die. Due to such high SPF temperatures, a lifetime of the tool (dies) is very short and energy consumption is extremely high. Therefore, a need to decrease the SPF temperature during the article manufacturing process down to 800° C. and below exists till the present time.
It is known that, in order to widen the temperature—strain rate interval during the SPF, α-phase grain sizes should be decreased (O. A. Kaybyshev. “Superplasticity of industrial alloys”. Moscow, ‘Metallurgy’ Publisher, 1984). Particularly, it is known at the present time that, in order to reduce the SPF deformation temperature, it is necessary to obtain a workpiece having submicrocrystalline structure (SMCS) with a grain size of 1 μm or lower (see “Forging production” in Russian, 1999, No. 7, pp. 17-19). The workpieces or semifinished products having such grain sizes would allow to reduce the SPF deformation temperature by several hundred degrees, depending on an alloying (doping) level of the alloys.
One of the most technically acceptable ways to obtain this workpiece structure is to use a polygonal (many-sided) isothermal forging method. There are some difficulties, however, in implementation of the presently proposed methods in production quantities using the currently existing equipment.
Also known is a method for processing metal and alloy billets by thermomechanical deformation in one or several steps, which method provides refining of billet material microstructure by choosing load conditions (see the Russian Patent No. 2,203,975, IPC7 C22F 1/18, which is issued May 10, 2003 and corresponds to the International patent application publication WO 01/81026 of Nov. 1, 2001). The load conditions provide microstructure transformation during a deformation and/or heat treatment process. Quantity of the deformation steps and the type of load are chosen taking into account configurations of the initial and final billets and grain size of the initial billet. At the first stage, the billet is obtained by multicomponent loading, in particular, by loading of “torque—tensile (compressive)” type. Further deformation of the billet is conducted in a sheath. This method allows to obtain the billets mostly of a round cross-section and a grain size less than 0.5 μm.
A major drawback of this method is a low process manufacturability, limited shapes and sizes of the produced billets. Realization of the process in production quantities requires great investment costs to provide necessary equipments and tools.
Thus, the above analysis of the current patent and literature prior art has proved a necessity to provide a technological method for manufacturing, in production quantities and with the use of currently existing equipment, big-sized semifinished products made of high-strength titanium alloys and having homogeneous submicrocrystalline structure.